JavaScript, output in calculator

• First you have no element with the id of submit.
• You are trying to add onclick handler to the element before it exists.
• You are selecting other elements with id, but they only have a name.
• You are adding a click event to a form, not sure what you expect that to do when there is no method output.

You are selecting by id, but the value arithmeticOperator is the name of your select box.

Try this: <select name="arithmeticOperator" id="arithmeticOperator">

document.getElementById gets elements by their id. You have multiple cases where you're trying to find elements with their name or type. That won't work.

document.getElementById('calcForm').onclick=output;
This tries to bind a function called output to the click event of an element called calcForm. I have no idea what you're trying to do here.

To show the result in an input or output, you can just do
document.getElementById("idOfTheElement").value = result;

As you seem eager to just know the solution - I've created this fiddle for you.

Please go over it carefully and check the edits I've done in your code.

Numerous errors, typos changed (as mentioned by others) include:

• Every name= instance can be swapped out by id= in your code.. It's crucial in javascript to understand the difference.
• You didn't declare any of the variables at the start. Any javascript variable starts with var (including arrays, objects and often even functions)
• The code was giving a POST error. You have to handle the submit event of a form (because HTML automatically wants to send it) - see the first line in the function event.preventDefault()

The javascript code reworked:

document.getElementById('submitButton').onclick = function(event) {
    event.preventDefault();

    var num1 = parseFloat(document.getElementById('valueA').value),
        num2 = parseFloat(document.getElementById('valueB').value),
        arithmeticOperator = document.getElementById('arithmeticOperator').value,
        output = document.getElementById('result');

    switch(arithmeticOperator) {
        case '+':
            result = num1 + num2;
            break;
        case '-':
            result = num1 - num2;
            break;
        case '*':
            result = num1 * num2;
            break;
        case '/':
            if(num2 != 0)
            {
                result = num1 / num2;
            }else {
                result = 'Dela inte med 0!';
            }
            break;
        default:
            result = 'Error';
    }
    output.innerHTML = result;
    window.alert(result);
    return false;
};

And the HTML with some modifications:

<form name="calcForm" id="calcForm">
    <p><input type="number" id="valueA" placeholder="Värde A" /></p>
    <p><input type="number" id="valueB" placeholder="Värde B" /></p>

    <select name="arithmeticOperator" id="arithmeticOperator">
        <option value="+">Addition</option>
        <option value="-">Subtraktion</option>
        <option value="/">Division</option>
        <option value="*">Multiplication</option>
    </select>
    <button type="submit" id="submitButton">Räkna ut</button>

    <p>Result: <output id="result" form="calcForm"></output></p>
</form>